Significant progress within the last 5 years has established that excitatory amino acid transmitters (EAAs) play a central role in the control of GnRH neurosecretion. The renewal proposal focuses on elucidating the mechanisms utilized by EAAs to regulate GnRH secretion, and further delineating the sequence/interaction of neuroendocrine signals in the induction of the preovulatory LH surge. Aim 1 will determine the role of nitric oxide (NO) as a downstream mediator of EAA effects in the hypothalamus to control GnRH and LH secretion. The effect of EAAs on hypothalamic NO production will be evaluated as will the essential question of whether NO production is regulated by steroids and is elevated at the time of the proestrus LH surge. Aim 2 will determine whether opioid neurons function to tonically inhibit hypothalamic glutamate and/or NO neurons by examining the effect of naloxone on hypothalamic glutamate release and NO production. Aim 3 will determine the precise second messenger systems utilized by glutamate and NO to regulate GnRH and LH release. Hypothalamic expression of the second messenger, guanylate cyclase and cyclooxygenase, will be determined during the cycle and the preovulatory LH surge, and following administration of glutamate and/or NO agents. Enzyme inhibitors will also be used to determine functional significance of these systems. Aim 4 will determine whether a defect in glutamate neurosignaling is involved in reproductive aging by determining whether EAA receptors, EAA release rates and NO production are attenuated in the hypothalamus of the middle-aged rat on proestrus afternoon. Aim 5 will establish whether the enzyme heme oxygenase, which synthesizes carbon monoxide in the brain, has a physiological role in production of the LH surge and is regulated by glutamate and/or steroids. Aim 6 will establish whether steroid hormones control hypothalamic glutamate transporter expression as a mechanism of regulating glutamate levels at the synapse. These studies will enhance our understanding of the complex neuroendocrine mechanisms controlling GnRH and LH secretion in the female.